1. Field of the Invention
This invention relates to a demodulation system for spread spectrum communication.
2. Description of the Prior Art
In typical spread spectrum communication, a transmitter side modulates an information signal with a spread code (a pseudonoise code, a PN code) and thereby converts the information signal into a spread spectrum signal. The spread spectrum signal is transmitted from the transmitter side to a receiver side. The receiver side generates a spread code which corresponds to the spread code used in the transmitter side. The receiver side despreads the received spread spectrum signal with the spread code to recover the information signal from the spread spectrum signal.
Spread spectrum communication has the following advantages. If a jamming signal is present in the frequency band of a spread spectrum signal, the spread spectrum signal is hardly affected by the jamming signal so that information components of the spread spectrum signal can be accurately maintained. When a plurality types of PN codes are used, spread spectrum signals can be multiplexed in a common frequency band.
Generally, in spread spectrum communication, it is necessary that an information signal is spread into a frequency band sufficiently wider than the original frequency band thereof. In fact, most of cases require a certain limitation on the frequency band of a spread spectrum signal. The limitation on the frequency band causes a distortion of the waveform of the spread spectrum signal. In a receiver side, such a waveform distortion adversely affects a despreading process, and thus a recovered information signal is contaminated by bit clock components of a PN code. The contamination of the recovered information signal results in a decrease in the information demodulation efficiency.